Various types of syringes are used for medical practices in medical facilities such as hospitals. Plastic disposable syringes have become mainstream recently. A syringe of this type that is used most commonly has a removable cap covering an injection needle. Keeping the injection needle covered with the cap until injection is started can prevent injuries and the like due to needlestick accidents before they occur. Covering the injection needle with the cap after the injection can prevent needlestick accidents and the like to occur during disposal of the syringe. The injection needle might be contaminated with a virus or the like infecting a patient after the injection, so that needlestick accidents should be prevented reliably particularly in such a case.
A syringe having an automatic housing function for an injection needle (needle) has been suggested in recent years with the intention of preventing needlestick accidents and the like to occur after administration of injection. As examples, a suggested syringe includes a needle retaining member biased toward a retreated side with a coil spring in a compressed state, and a restricting member to restrict retreat of the needle retaining member (see patent literatures 1 and 2, for example). According to such a syringe, pushing in a plunger (injection piston) further after injection deforms or displaces the restricting member to release restriction. This makes the needle retaining member retreat to house an injection needle. As another example, a suggested syringe includes a needle retaining member biased toward a retreated side and a latch mechanism to restrict retreat of the needle retaining member (see patent literature 3, for example). According to this syringe, the latch mechanism is disengaged when injection is completed, thereby making the needle retaining member retreat to house an injection needle.
However, the aforementioned conventional syringe having the automatic housing function for the injection needle encounters the following problem. The needle retaining member retaining an injection needle is biased with the spring. Hence, for assembly of the needle retaining member, assembling work should be accompanied by compressing the spring gradually, so that favorable workability of assembly might become unfeasible.